There are several power converter topologies that have been developed over the years, which are intended to improve the power density and switching efficiency of power converters. An emerging focus of new converter topologies is to provide a means to reduce or eliminate converter switching losses, while increasing the switching frequencies. Lower loss and higher switching frequency means more efficient converters, which can reduce the size and weight of converter components. Additionally, with the introduction of high speed composite semiconductor switches, such as metal oxide semiconductor field effect transistor (MOSFET) switches operated by pulse width modulation (PWM), recent forward and flyback topologies are now capable of operation at greatly increased switching frequencies, such as, for example, up to 1.0 MHz.
However, an increase in switching frequency can cause a corresponding increase in switching and component stress related losses, as well as increased electromagnetic interference (EMI), noise, and switching commutation problems, due to the rapid ON/OFF switching of the semiconductor switches at high voltage and/or high current levels. Moreover, modern electronic components are expected to perform multiple functions, in a small space, efficiently, and with few undesirable side effects. For instance, a modern voltage converter that provides for relatively high power density and high switching frequencies, should also include uncluttered circuit topologies, provide for isolation of the output or “load” voltage from the input or “source” voltage, and also provide for variable step-up or step-down voltage transformation.
FIG. 1 illustrates a conventional flyback type voltage converter. The converter 10 includes a transistor T1, a controller 14, a transformer 12, a capacitor C1, and a diode D1. Input voltage to the circuit may be unregulated DC voltage derived from an AC supply after rectification and filtering. The transistor T1 is a fast-switching device, such as a MOSFET, the switching of which is controlled by a fast dynamic controller 14 to maintain a desired output voltage Vout. The secondary winding voltage is rectified and filtered using the diode D1 and the capacitor C1. The transformer 12 of the flyback converter functions differently than a typical transformer. Under load, the primary and secondary windings of a typical transformer conduct simultaneously. However, in the flyback converter, the primary and secondary windings of the transformer do not carry current simultaneously. In operation, when the transistor T1 is turned ON, the primary winding of the transformer 12 is connected to the input supply voltage such that the input supply voltage appears across the primary winding, resulting in an increase of magnetic flux in the transformer 12 and the primary winding current rises linearly. However, with the transistor T1 turned ON, the diode D1 is reverse biased and there is no current through the secondary winding. Even though the secondary winding does not conduct current while the transistor T1 is turned ON, the load, represented as resistor Rload, coupled to the capacitor C1 receives uninterrupted current due to previously stored charge on the capacitor.
When the transistor T1 is turned OFF, the primary winding current path is broken and the voltage polarities across the primary and secondary windings reverse, making the diode D1 forward biased. As such, the primary winding current is interrupted but the secondary winding begins conducting current thereby transferring energy from the magnetic field of the transformer to the output of the converter. This energy transfer includes charging the capacitor C1 and delivery energy to the load. If the OFF period of the transistor T1 is sufficiently long, the secondary current has sufficient time to decay to zero and the magnetic field energy stored in the transformer 12 is completely dissipated.
The flyback topology has long been attractive because of its relative simplicity when compared with other topologies used in low power application. The flyback “transformer” serves the dual purpose of providing energy storage as well as converter isolation, theoretically minimizing the magnetic component count when compared with, for example, the forward converter. Increasing the switching frequency is an effective way of reducing the size and weight of the overall power supply. A drawback to use of the flyback is the relatively high voltage and current stress suffered by the switching components. Additionally, high turn-off voltage (caused by the parasitic oscillation between transformer leakage inductance and switch capacitance) seen by the primary switch traditionally requires the use of a resistor, capacitor, diode subcircuit, such as a snubber circuit. This parasitic oscillation is extremely rich in harmonics and pollutes the environment with EMI, and causes high switching losses from the switching components in the form of extra thermal dissipation.
In an effort to reduce or eliminate the switching losses and reduce EMI noise the use of “resonant” or “soft” switching techniques has been increasingly employed in the art. The application of resonant switching techniques to conventional power converter topologies offers many advantages for high density, and high frequency, to reduce or eliminate switching stress and reduce EMI. However, the complexity required to provide control to the power switches, such as the transistor T1 in FIG. 1, and the components associated with complex controls, create a limited use in commercial applications.
Resonant switching techniques generally include an inductor-capacitor (LC) subcircuit in series with a semiconductor switch which, when turned ON, creates a resonating subcircuit within the converter. Further, timing the ON/OFF control cycles of the resonant switch to correspond with particular voltage and current conditions across respective converter components during the switching cycle allows for switching under low or zero voltage and/or low or zero current conditions. Zero voltage switching (ZVS) and/or zero current switching (ZCS) inherently reduces or eliminates many frequency related switching losses.
The application of such resonant switching techniques to conventional power converter topologies offers many advantages for high density, high frequency converters, such as quasi sinusoidal current waveforms, reduced or eliminated switching stresses on the electrical components of the converter, reduced frequency dependent losses, and/or reduced EMI. However, energy losses incurred during control of zero voltage switching and/or zero current switching, and losses incurred during driving, and controlling the resonance means, are still problematic.
In the flyback converter that is commonly used in small power products, such as a charger or adaptor power supply, the converter operates in a discontinuous mode, which means the current through the primary winding of the transformer is allowed to fall to zero. In one approach, this converter utilizes the resonance which occurs after the freewheeling current drops to zero to discharge the drain to source parasitic capacitance of the switching transistor. While the resonance voltage reaches a valley point, the transistor is turned ON to achieve low voltage turn on, and even zero voltage turn on at certain conditions. The valley voltage level achieved during resonance depends on the input voltage level and the reflected output voltage on the primary winding of the transformer. However, the lower the valley voltage level, the higher the voltage spike across the transistor when the transistor is switched OFF. As such, the valley voltage level is limited by the corresponding stress applied to the transistor when switched OFF. The stress to the transistor increases with increasing input voltage.
In another approach, a capacitor is added across the freewheeling diode on the secondary side of the flyback converter. After freewheeling current drops to zero, the reverse voltage across the diode charges the capacitor. The charge current reflects to the primary side of the circuit and generates a discharge current applied to the transistor, which pulls down the drain to source voltage. The transistor drain to source voltage can be zero at turn on as long as the discharge current is high enough. However, the resonant current between the transformer and the added capacitor across the freewheeling diode increases the current stress on the secondary winding and leads to additional conductive dissipation.